jailsetup.sh: set default diff.renameLimit=250

jailsetup.sh: set default diff.renameLimit=250

Over time Git has had various defaults for the value of `diff.renameLimit`.

The documentation neglects to mention what the default might actually be.

In Git 1.6.6 (the oldest version still supported by Girocco), the default
limit is 200.  As of Git 1.7.5 (2011-04-23) the default limit was increased
to 400.

Avoid the confusion and set a consistent default in the Girocco-global
config file ($Girocco::Config::chroot/etc/girocco/.gitconfig), but only if
one is not already set in there.

In other words, it can easily be changed using the
toolbox/girocco-config.sh utility script and it will stick.

If `diff.renameLimit` does not currently exist in the Girocco-global config
file, set it to 250 at install time to provide a consistent default.

Individual projects can always override it if there's some specific need.

A value of 250 should provide adequate functionality in most cases without
unnecessarily overburdening the server especially considering that Git
"gracefully degrades" the functionality,

In other words, it will drop "--find-copies-harder" if there are too many
candidates for the limit and then drop "-C" if there are still too many
candidates for that.  Git should always, however, find exact matches
(where the blob hashes remain the same) no matter what limit is used.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>